1. Field of the Invention
The present invention relates to a method of producing single crystals of the gadolinium molybdate family, more particularly the single crystals having a high transmission and a low threshold field.
2. Description of the Prior Art
The single crystals of gadolinium molybdate family are generally ferroelectric and ferroelastic and have a high transmission. Consequently, these crystals provide very excellent characteristics when they are used as an optical material.
And it is well known that in order to obtain the single crystals of gadolinium molybdate family having good quality, a technique of growing a single crystal from a melt at high temperature according to a pull technique (which is generally known as "Czochralski method") is used.
In this technique, the single crystals of gadolinium molybdate family, for example, a gadolinium molybdate (Gd.sub.2 (MoO.sub.4).sub.3) single crystal, are produced by heat-melting a polycrystal of gadolinium molybdate or a mixture consisting of molybdenum oxide and gadolinium oxide which is so compounded as to satisfy a stoichiometric value as constitutes gadolinium molybdate, by immersing a seed crystal in the resultant melt, by growing the single crystal of gadolinium molybdate with the crystal pulling technique, and thereafter by cooling the single crystal from a high temperature to room temperature.
But, if the cooling rate in the cooling process is low, then the crystal becomes whitish, becomes low in light transmission and cannot be put to an optical application.
With note taken of the relationship between the cooling rate and the whitening of the crystal, the inventors have previously proposed a method in which the cooling rate in the cooling process after growing the gadolinium molybdate single crystal from the melt by the crystal pulling technique is made large, thereby to produce an optical material of sufficiently high transmission.
On the other hand, when the gadolinium molybdate single crystal is to be employed as the material of an electro-optical element such as optical shutter, light modulator and color modulator, it is desired that besides a high transmission that the threshold field is low while the mobility is high. Herein, the terms "threshold field" and "mobility" mean the point (1) of intersection with the axis of abscissas as obtained by the extrapolation and the inclination tan .theta. of a characteristic straight line, respectively, in the characteristic of electric fields - transfer velocity of a polarization wall (wall velocity) as shown in FIG. 1.
But the gadolinium molybdate single crystal obtained by the process mentioned above has a large threshold field, therefore it cannot be used in an electro-optical application.